In integrated circuit manufacturing, plasma chambers are used to process substrates. A plasma chamber is typically coupled to a radiofrequency (RF) source to provide energy to ignite and/or maintain a plasma during substrate processing. To effectively couple RF energy to the chamber, a tunable matching circuit (also referred to as a matching unit) is connected between the RF source and the plasma chamber.
In one type of system, the RF source has a fixed frequency and matching circuit has two tunable reactors (e.g., tunable vacuum capacitors). The tunable reactors are tuned using stepper motors to provide two tuning variables for achieving and maintaining a match condition. Unfortunately, the tunable reactors are very expensive and the use of stepper motors to tune two reactors can require a significant amount of time to achieve and maintain the match condition.
To facilitate removal of one of the tunable reactors from the matching circuit, a tunable frequency RF source may be used. Consequently, one tuning variable is the tunable RF frequency and a second variable is a tunable reactor in the matching circuit. When using a tunable frequency RF source, a control system determines the chamber impedance by monitoring the phase and magnitude of the RF current and voltage being applied to the chamber. Such monitoring may be accomplished using a quadrature demodulator or digital signal processing. The RF source frequency and the matching circuit are adjusted to effectively match the RF source impedance to the estimated chamber impedance. Such a match maximizes the amount of RF energy that is coupled from the RF source to the plasma chamber. As chamber conditions vary, the matching circuit reactor value and source frequency are adjusted to maintain the impedance match.
The impedance analyzer (e.g., the quadrature demodulator) is driven with a fixed clock signal that is unrelated to the RF frequency being applied to the chamber and produces a control signal based solely upon the measurement of the applied RF voltage and RF current. Such impedance estimation can lead to inaccurate match circuit control and, at times, the inaccurate match circuit operation may lead to substrate damage or chamber damage.
Therefore, there is a need in the art for a method and apparatus for improving substrate processing chamber control.